1. Field of the Invention
This invention relates to a fine-denier, high-tenacity, water-resistant polyvinyl alcohol synthetic fiber and a method for its production.
2. Description of the Prior Art
A typical use for fine-denier fiber is synthetic leather. The principal method for producing such fine-denier fiber comprises mix-spinning two incompatible, heat-fusible polymer chip materials to prepare an "oceano-insular" composite fiber, processing the fiber into a two-dimensional article such as a nonwoven fabric, and extracting out the oceanic phase (sea component) of said fiber to leaving the insular phase (island component) alone and providing a shaped article made up of fine-denier fiber. Although this production method is suitable for applications in which a two-dimensional structure cannot be easily fabricated directly from fine-denier fiber, the fine-denier fiber obtainable by such a method is low in strength, and though useful for synthetic leather, is inadequate for use as an industrial material. Moreover, as the method includes a step of extracting and removing said oceanic phase, it entails wastes in materials and procedures, with the result that the cost of the product fine-denier fiber is necessarily high.
Attempts have also been made to manufacture fine-denier fiber from polyvinyl alcohol (hereinafter referred to abbreviated form as PVA).
Japanese Patent Publication No. 31376/1972, for instance, teaches the art of manufacturing a fine-denier fiber for papermaking use which comprises mix-spinning completely saponified PVA with sparingly saponified PVA, stretching and heat-treating the resulting tow to give an ordinary-denier fiber, and beating the same to give a fibril-like, fine-denier fiber for papermaking use. In this method, as it employs a partially saponified PVA with a large side chain, the mix-spun fiber is difficult to stretch and, moreover, crystallization is markedly inhibited. Therefore, while the fiber is low in strength and water resistance even before the beating process, the mechanical beating disturbs the orientation of molecules in the amorphous phase and of crystals and even destroys the crystals so that the strength and water resistance are still further sacrificed.
Japanese Patent Publication Kokai Sho 54-77720 discloses a method of producing PVA fine fiber. This method, too, is a technique for mix-spinning a highly saponified PVA with a partially saponified PVA, and the characteristic feature of this technique lies in the removal of the low-saponified PVA from the resulting ordinary-denier fiber by aqueous washing. The difference between this method and the preceding method is that whereas the means used for converting ordinary-denier fiber into fine-denier fiber in the preceding method is that of removing the low-saponified PVA with the aid of mechanical beating so as to achieve fibrilization, the means employed in the latter method causes the low-saponified PVA to swell considerably and be washed off with water. In either method, the resulting fine-denier fiber is low in tenacity and water resistance.
The method described in Japanese Patent Publication No. Sho 58-38526 is also similar to the above-mentioned methods in that it employs a PVA of low polymerization degree as the partially saponified PVA and the fiber obtainable by the method is also low in tenacity and water resistance. For example, the fine PVA fibril shown as a working example of the patent application is of low tenacity. Even in the form of an ordinary-denier fiber prior to aqueous washing it has a low tenacity of only 3.4 g/denier.
Japanese Patent Publication Kokai Sho 54-30930 teaches a method comprising the use of an amorphous water-soluble polymer in lieu of a low-saponified PVA but the method is essentially the same as the above-mentioned methods.
In any event, the prior art comprises mix-spinning a crystalline PVA which is to become a strength fraction and a polymer which is low in crystallinity and readily soluble in water and removing the readily soluble fraction from the resulting ordinary-denier fiber to give a fine denier fiber. The fibers according to the foregoing methods are not only invariably low in tenacity and water resistance but also require a step for removing a component thereof in their production sequence, the component removed representing a loss of material. In other words, these fibers are of necessity costly.
Recently there has been an increasing need for fine-denier PVA fiber of high tenacity, high water resistance and low production cost. A typical application for such fiber is that of reinforcing brittle materials such as cured cement and low-strength plastic materials such as some synthetic resins.
For a fiber to be useful as a reinforcement, not only high strength but also the adhesivity of the fiber to the matrix material is an important factor. Reducing the diameter (denier number) of such fiber means an increased available area of contact with a matrix and, hence, an improved adhesivity to enhance the reinforcing effect. Moreover, the moldability of the product is also remarkably improved.
When the matrix is a hydraulic material, water resistance is an important requirement. Thus, since the fiber is exposed to water of comparatively high temperature during setting, it should not swell or decrease in strength under such conditions.
The ordinary PVA fiber has been used in various industrial materials for reasons of its high tenacity. As this fiber has a very high Young's modulus, the yarn, cord, rope, net, fabric and other products made thereof are characteristically high in rigidity and this is a disadvantage in certain applications. For such uses, it is effective to reduce the denier number of monofilaments. Moreover, reducing the denier number leads to an improved tenacity utilization rate which in turn results in an increased strength of products.
Furthermore, in recent years, germ- and dust-free rooms or workplace have been required in hospitals and in the electronics industry, for instance. To meet this requirement, high-performance filters are strongly demanded. For this purpose, nonwoven fabrics made of fine-denier fiber are effective.
As mentioned above, a high-tenacity, fine-denier, water-resistant PVA fiber is desired.